Field of the Invention
The present invention relates to controlling a generator system connected to an electric power system so as to avoid the unintentional islanding of the generator. More particularly, the present invention relates to actively detecting generation islands using a combination of frequency characteristic thresholds, and an active phase angle destablization technique to destablize well or perfectly matched islands.
Many businesses, manufacturing companies, homeowners, etc. use generators in addition to power delivered by the local electric power company (also referred to as an electric power system or utility grid). However, non-utility owned generator systems connected to an electric power system create both operational and maintenance problems.
The operational problems include a non-utility owned generator system not being synchronized with a de-energized power grid included in the electric power system. The lack of synchronism between the non-utility owned generator system and the electric power system cause a higher than normal voltage across open isolation devices included between the non-utility owned generator and the electric power system, as well as higher than normal current flow when the isolation devices are closed. The higher than normal voltages across the opened isolation devices damages the devices and the higher than normal current flows tend to prematurely open over-current protection devices associated with the non-utility owned generator system and the electric power system.
Maintenance problems include personnel inadvertently contacting portions of the electric power system which are energized from the non-utility owned generator system. This is a severe problem which often results in injury or even death. For example, during a severe winter storm, utility companies have to dispatch emergency crews throughout neighborhoods to repair downed transmission lines, etc. To safely repair the downed lines, isolation devices corresponding to a power grid including the downed transmission lines are intentionally opened so the downed lines and associated transmission components are de-energized. However, if a homeowner plugs in a portable generator system to provide electricity for his family, for example, the power from the portable generator system may be fed back into the de-energized power lines, transformers etc, critically injuring a maintenance worker.
To avoid these types of possibly fatal injuries, the utility company would have to manually isolate all portable generator systems, etc., connected to the local grid requiring maintenance so as to bring the voltage to a safe level before beginning any maintenance work. This is extremely ineffective and time-consuming. In fact, it is virtually impossible to know when and where every portable generator system will be used. Thus, the power companies require generator systems connected to the utility grid to include protective devices.
The present invention is directed to solving the above and other noted problems.
To solve these problems, the present invention provides a novel method of controlling a generator system connected to an electric power system in which an output current phase angle of the generator system is varied, and an output frequency characteristic of the generator system is measured. Further, the method determines whether or not the generator system is within a generation island based on the measured frequency characteristic
In another method, the output frequency characteristic of the generator system is measured, a first phase angle of the measured frequency characteristic is estimated using a first phase locked loop having a first bandwidth, and a second phase angle of the measured frequency characteristic is estimated using a second phase locked loop having a second bandwidth greater than the first bandwidth. Further, a phase shift between the estimated first and second phase angles is calculated, and the method determines whether or not the generator system is within a generation island based on the calculated phase shift.
In still another method, the output frequency characteristic of the generator system is measured, a first phase angle and frequency of the measured frequency characteristic is estimated using a first phase locked loop having a first bandwidth, and a second phase angle and frequency of the measured frequency characteristic is estimated using a second phase locked loop having a second bandwidth greater than the first bandwidth. Further, the method calculates a frequency difference between the first and second estimated frequencies, and an angle variation that is proportional to the calculated frequency difference. The estimated second phase angle is then added to the calculated angle variation so as to form an output current phase angle reference, and an output current phase angle of the generator system is controlled to be aligned with the output current phase angle reference. The method also determines whether or not the generator system is within a generation island based on the measured frequency characteristic.
The present invention also provides novel computer program products coded to execute the above methods within a generator system.